Polyvinyl chloride (PVC) shows a unique behavior among polymers in that a stable fluid suspension of emulsion or micro-suspension polymerized polyvinyl chloride in a plasticizer (which suspension is known in the art as a "plastisol") can be formed by adding very fine particles of PVC to the plasticizer with relatively simple mixing techniques. When heated above about 120.degree. C., the plastisol forms a homogeneous rubbery polymer product that is similar to the one obtained by the traditional melt extrusion. This process is called gelation and is amply described in the literature, e.g. PVC Technology (4th Edition), W.V. Titow (Elsevier).
A characteristic feature of PVC is that it does not contain sites of suitable reactivity to enable it conveniently to be crosslinked by reaction with common reagents. Since plasticized polyvinyl chloride is not a crosslinked structure it is prone to flow under sustained stress which does not occur with conventional crosslinked rubbers. Furthermore, the rate of flow of PVC increases more rapidly with increasing temperature than is normally true for crosslinked rubbers. In more practical terms, this means that a highly plasticized polyvinyl chloride body often suffers from excessive creep and stress relaxation, when subjected to sustained stress or strain, especially at elevated temperatures. It is, therefore, unsuitable for applications where sustained stress or strain are to occur unless fabric or other physical reinforcement is provided to restrict irreversible flow.
Another consequence of PVC not solidifying by crosslinking is that it swells in polar solvents to the extent that it completely dissolves in a few of them, tetrahydrofuran (THF) and cyclohexanone being the best known. A further limitation to the use of plasticized PVC has been the tendency of most plasticizers to exude out of a PVC composition over time.
These problems are quite serious and have generally restricted the use of flexible PVC to room temperature applications, such as artificial leather, floor covering, carpet backing, gloves, toys, etc. In glazing applications, there are also limitations against using flexible PVC gaskets, because when the PVC is in direct contact with, for example, Plexiglas.RTM. or polycarbonate glazing sheets, plasticizer migration often causes microcracks in the glazing sheet, thus reducing its mechanical resistance. These and other difficulties experienced with the prior art forms of PVC plastisols have been obviated in a novel manner by the present invention.
U.S.S.R. Inventors Certificate 931,732 discloses plastisol compositions containing only 40-60% PVC, 1-3% polyethyleneglycol, 0.5-2% polyhexamethylene diisocyanate, and the balance dioctylphthalate. With a maximum of only 5% polyurethane, the long term stability of the composition is not effected thereby.
It is, therefore, an object of the invention to provide an article of a solidified PVC plastisol composition which exhibits the characteristics of a vulcanized (crosslinked) polymer, including non-melting and reduced swelling in solvents as compared to a conventional PVC articles.
Another object of this invention is the provision of a process for forming a modified PVC product having an improved resistance to heat and solvents.
A further object of the present invention is the provision of a process for forming a modified PVC which exhibits reduced plasticizer migration.
With the foregoing and other objects in view, which will appear as the description proceeds, the invention resides in the combination and arrangement of steps and the details of the composition hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.